I'm still following this discussion with interest and I would bet my pension that had there been no tree to start with, NOBODY would be coming into this thread saying "great work, but it really needs a tree to give it added dimension". Nobody. They'd just be saying "great work", and waxing lyrical about the excellence of the figures and the groundwork (and rightly so).
I also agree with Colin that the red "tree" is like no tree that I have ever seen in nature. To me it looks more like a diagram of a network of veins and arteries out of some medical textbook. The vignette is much better without it IMHO.
- Steve